Many devices have been developed for monitoring a person's performance while exercising. For example, GPS devices have been used while running, biking, and swimming to track speed and distance among other parameters. These parameters are calculated using a series of GPS coordinates determined by the device along with a time when each of the GPS coordinates is determined.
GPS devices are more commonly used while running and biking because the information displayed by such devices can be easily viewed during these activities (e.g. because the GPS device can be worn on the arm or mounted to the bike which can remain stationary without impacting the performance of the activity). While swimming, however, it is difficult to view the information provided by a GPS device. For example, while swimming, a person cannot stop the motion of his arm without significantly affecting his performance. For this reason, GPS devices designed for swimmers are oftentimes configured to be worn on the head. Also, some recent GPS devices have been designed to output some information for display in the lens of the swimmer's goggles.
Open-water swimming also presents a problem that is not present when running or biking. When swimming in the open water, it is difficult to swim in a straight line towards an intended destination. For example, some swimmers naturally swim in a curved line due to imbalances in their stroke. Currents in the open water can also cause a swimmer to swim in an undesired direction. To address the difficulty of swimming in a straight line, many swimmers will use a sighting technique. When sighting, a swimmer will periodically lift his head out of the water in an attempt to see a landmark that he is using as a guide. In this way, the swimmer can determine if he has deviated from his intended path and can adjust his direction accordingly.
Sighting poses various difficulties. For example, because the swimmer must lift his head, sighting tends to slow the swimmer's speed and increases the exertion required to reach a destination. Also, when no obvious landmark exists or high waves make it difficult to see a landmark, it can require more time to locate a landmark during each sighting. This additional time can cause the swimmer to lose some or even all his momentum (e.g. when his feet sink into the water) further slowing his speed and increasing the exertion required. Further, if the swimmer is consistently deviating from a straight line path, the swimmer may swim a significantly longer distance than intended. Swimming this extra distance can be very undesirable in races such as triathlons both because of the additional time required to swim the extra distance as well as the additional energy exerted to cover the extra distance.